Messagerie is indeed translated as “parcel service” or “small shipment” but in a French railway context it refers to a train category, notably regarding braking characteristics. Messagerie (ME or MV) trains are shorter, have lower axle loads, and the major difference with marchandises (MA) trains is that they operate under braking rules similar to (loco-hauled) passenger trains. Messagerie trains operate up to speed limits of 160 km/h (MV160) (*) while the fastest Marchandises trains are limited to 100 km/h (MA100).
Messagerie trains are most of the time composed of piggyback flatcars (“caisses mobiles” or containers), but can also be composed of car-carrying wagons, refrigerated wagons (as alluded to above), etc.
(*) MVGV trains were discontinued in the early 2010s for several reasons (the cost of maintaining a small fleet of TVM-equipped locomotives and 200 km/h-capable wagons was too high and the overall time saving on the Valenton-south of France journey brought by the use of the LGV was not that high)